We propose to investigate the role of the Epstein-Barr virus (EBV) and human herpesvirus 6 (HHV- 6) in the etiology of multiple sclerosis (MS) among participants in two large prospective cohort studies: the Nurses' Health Study (NHS) and the Nurses' Health Study II (NHS II). Our study population comprises over 230,000 women, including 62,547 who have provided blood samples for serological and genetic analyses and over 20,809 who have provided mouthwash samples. Over 400 incident cases of MS have been documented in this cohort and we project a total of 600 incident cases by the end of the follow-up. We have previously reported that healthy women with elevated serum liters of anti-EBV antibodies have a significantly increased risk of developing MS. These serological results, however, are insufficient to establish causality. We propose therefore to apply molecular methods to elucidate the biological mechanisms that may relate MS to the EBV. As part of the proposed investigation we will determine the EBV viral load in cases and controls, and examine the roles of genetic variations in the host (HLA-DR and DQ and other candidate gene polymorphisms) and the virus (variations in the EBNA-1, LMP-1, EBNA-2, and EBNA-3c genes). Futher, we will examine whether elevated plasma titers of anti-HHV-6 antibodies or detectable plasma levels of HHV-6 DNA predict the risk of MS. Strengths of the proposed investigation include sampling from two well-characterized cohorts that have already contributed several findings on the epidemiology of MS, a control group randomly chosen from the population that generated the cases, and the availability of blood samples collected before the onset of MS in a subset of cases. Most important is the interdisciplinary approach of the proposed project, which includes collaboration with experts in EBV virology, immunology, and MS genetics. [unreadable] [unreadable] [unreadable]